Re-add accidentally deleted doc comment.

sdk/lib/_internal/pub/lib/src/barback.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library pub.barback;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
@@ skipping 39 matching lines @@
   /// This is relative to the `lib/` directory in [package], and doesn't end in
   /// `.dart`.
   ///
   /// This can be null; if so, it indicates that the transformer(s) should be
   /// loaded from `lib/transformer.dart` if that exists, and `lib/$package.dart`
   /// otherwise.
   final String path;
 
   /// The configuration to pass to the transformer.
   ///
-  /// This will be an empty map if no configuration was provided.
+  /// Any pub-specific configuration (i.e. keys starting with "$") will have
+  /// been stripped out of this and handled separately. This will be an empty
+  /// map if no configuration was provided.
   final Map configuration;
 
   /// The primary input inclusions.
   ///
   /// Each inclusion is an asset path. If this set is non-empty, than *only*
   /// matching assets are allowed as a primary input by this transformer. If
   /// `null`, all assets are included.
   ///
   /// This is processed before [excludes]. If a transformer has both includes
   /// and excludes, then the set of included assets is determined and assets
@@ skipping 234 matching lines @@
 
   if (id.package != entrypoint) {
     throw new FormatException(
         'Can only access "lib" and "asset" directories of non-entrypoint '
     'packages.');
   }
 
   // Allow any path in the entrypoint package.
   return path.url.join("/", path.url.joinAll(parts));
 }